1. Field of the Invention
The present invention relates to a method for producing a microbial polyester. More particularly, the present invention is concerned with a method for producing a microbial polyester comprising 3-hydroxybutyrate monomer units, in which a strain belonging to the genus Alcaligenes is cultured in a liquid medium. The microbial polyester produced is useful in plastics and polymers which are free from environmental pollution problems, and in implanting materials and drug carriers, recovery of which is not necessary.
2. Discussion of Related Art
Microbial polyesters, such as poly(3-hydroxybutyrate), which are produced by the biosynthetic function of a microorganism, are readily biodegraded by microorganisms and within the body of higher animals, including humans, whereas it is generally impossible to biodegrade petrochemically synthesized polymers. The poly(3-hydroxybutyrate) produced by the biosynthetic function of a microorganism is entirely composed of optically active D-(-)-3-hydroxybutyrate monomer units (optical purity 100%), and readily biodegraded. On the other hand, polymers entirely composed of optically active monomer units cannot be prepared by the present petrochemical technology, and polymers prepared by the present petrochemical technology are generally not biodegradable. In recent years, tremendous amounts of synthetic plastics and polymers have been used in various application fields, and tremendous amounts of used plastics and polymers have been piled as bulky undecayed waste. Pollution of the environment by such bulky undecayed waste is now a serious problem. The above-mentioned microbial polyesters are introduced in the ecosystem through biodegradation thereof, so that they are useful as plastics and polymers which are free from environmental pollution problems. Moreover, in the field of medicine, the microbial polyesters can be used as an implanting material and a drug carrier, recovery of which is not necessary.
It has been reported that a wide variety of bacteria produce poly(3-hydroxybutyrate) in their cells, accumulating the same in the form of particles (see H. Brandle et al., Adv. Biochem. Eng./Biotechnol., 41, pages 77-93, 1990). In particular, it has been reported that copolymers comprised of 3-hydroxybutyrate monomer units and 3-hydroxyvalerate monomer units or comprised of 3-hydroxybutyrate monomer units, 3-hydroxyvalerate monomer units and other monomer units are produced from a preselected water soluble carbon source, such as glucose, propionic acid, butyric acid, or combinations thereof, by the use of Alcaligenes eutrophus (see Japanese Patent Application Laid-Open Specification No. 57-150393/1982), strains belonging to any one of the genus Nocardia, the genus Corynebacterium and the genus Rhodococcus (see European Patent No. 396,289) and Rhodospirillum rubrum (see H. Brandl et al, Int. J. Biol. Macromol., 11, pages 49-55, 1989). Commercial use of the above-mentioned copolymers comprised of 3-hydroxybutyrate monomer units and 3-hydroxyvalerate monomer units is being promoted as a thermoplastic resin improved in hardness and brittleness as compared to those of poly(3-hydroxybutyrate).